1. Field of the Invention
The present invention is in the field of gas analysis and more particularly relates to a compact apparatus for measuring the absorption of light or other radiation by a gas that absorbs only weakly.
2. The Prior Art
Certain gases have absorption bands that absorb so weakly that absorption can only be detected after the radiation has traveled a relatively long distance, perhaps kilometers, through the gas. On the other hand, practical gas analyzers for commercial use, as opposed to laboratory apparatus, typically are small enough to be portable, and are definitely too small to provide path lengths thousands of meters long.
Furthermore, the amount of gas available might be insufficient to fill a sample chamber large enough to provide the necessary path lengths.
It is well-known to use mirrors to fold an optical beam so that the beam can traverse the sample cell a number of times. Although the present invention makes use of a folded optical path, that alone is not the inventive step.
A multi-path absorption cell was described by J. U. White, Journal of the Optical Society of America, Volume 32, page 285 (1942). The essential parts of the White cell consist of three spherical concave mirrors all having the same radius of curvature, and positioned to form an optical cavity. Utilizing the principle outlined in White's article, at least two companies have marketed a ten-meter multi-path cell, and one of the companies has also marketed a forty-meter multi-path cell.
The number of times the light can be passed through a White cell is limited by the spherical aberation of the mirrors. Thus, the White cell is not compatible with the present invention in which the light may be passed through the sample cell thousands of times.